Let us begin by making clear just what _is_ the ideal which guides the scientist in his expectations respecting the world he studies. Perhaps no one accomplishment of science has been more inspiring than the picture of certain large aspects of nature that Newton succeeded in drawing,--such aspects, namely, as are presented in the behavior of suns, and planets, and moons. All these huge masses are governed by a single law, called the law of gravitation. Now to say that they are governed by this law means no more than this, that if we knew the mass, the position, and the velocity of each of these heavenly bodies at a given moment, we should be able by means of the law to predict their masses, positions, and velocities for all future moments. This result is, to be sure, only an approximation, for we know that gravitation is not the only force which bodies exert on each other. We have never succeeded, _e.g._, in reducing the attractions and repulsions of electrified bodies to gravitation, nor do we any longer try to do this. But Newton’s degree of success provides us with an ideal to which we seem ever more and more closely to approach. Instead of considering such huge bodies as suns and planets and their satellites, we divide these up into extremely minute parts, which we may call for the moment _atoms_. We struggle then to conceive a law as completely determining the behavior of these atoms, as the law of gravitation determines that of planets. So that, if we knew a limited number of characteristics of each of these atoms at a given moment, our law would enable us to predict their future and to reconstruct their past history. As we approach more and more closely to this ideal, less and less in the behavior of these small parts of nature is left to guess-work. In so far as we hope this ideal may be continuously approached, we hope that in its atomic parts nature is entirely devoid of freedom. And if we hope this, we must inevitably hope, too, that what we have called an atom is neither a moral nor an immoral being. This hope is usually called _the mechanical ideal_, and nature in the light of it is viewed as a mechanism. It has guided science to victory after victory, and I venture to think that no result of philosophical experience is more firmly established than this, that whatever theory we may in the end accept respecting human nature, its freedom, its moral responsibilities, no assumption of that theory may stand in contradiction with the mechanical ideal. To have recognized this truth and to have had the courage to maintain it at all costs, was the heroic service rendered by Spinoza at a moment in human history when such service was badly needed. It is also the reason why Spinozism, in spite of its apparently gloomy outlook upon the world, has made such a forcible and lasting appeal to the imagination of thinking men. In what follows it is against certain false implications that have been thought